1. Field of the Invention
The present invention relates to a communication control apparatus and method for setting the parameters for a communication apparatus in a base station to optimal values based on an estimated moving speed of a mobile station in a mobile communications system.
2. Description of the Related Art
Conventionally, for a channel multiplex method in a mobile communications system, a time division multiple access (TDMA) system, a frequency division multiple access (FDMA) system, etc. have been used. However, the development of a method with a more efficient use of frequency is desired, and particularly, a direct sequence code division multiple access (DS-CDMA) system is a promising candidate, since a large volume of communications is available in this system.
The DS-CDMA system is one kind of a spread spectrum communication system. In this system, on the transmitting side, data signals are transmitted with the spread spectrum by using the same frequency for a plurality of channels and multiplying the data signals by an independent spread code with a broad band for each channel. On the receiving side the data signals for each channel are restored by multiplying received signals by the same spread code. The multiplication of a spread code on the receiving side is called a reverse-spread. When this DS-CDMA system is applied to mobile communications, a searcher function, a transmission power control function, a coherent detection function, etc. are indispensable features.
A searcher function means a function for detecting a transmission path and a despread code timing being a timing for performing a reverse-spread. A transmission power control function means a function for modifying a transmission power against a distance problem due to the difference in the distance between a mobile station and a base station, and an instantaneous fluctuation (fading) due to multi-paths. A coherent detection function means a function for adding a pilot signal to data signals in order to obtain a required bit error rate (BER) in a lower transmission power when transmitting, and performing a coherent detection.
In mobile communications a stable communication is required in a variety of dynamically changing environments, such that a mobile station may transit from a stationary status to a high-speed status, that a mobile station may transit from an urban environment to a suburban environment, etc. Particularly, in a multi-path environment with reflected waves and delayed waves passing through a plurality of transmission lines, countermeasures are indispensable, since fading (instantaneous value fluctuation) is generated by such interference. In the DS-CDMA system, effective fading countermeasures are also desired in connection with each of the above-mentioned functions.
However, there are the following problems in mobile communications using the DS-CDMA system.
Generally speaking, although there are optimal values for parameters for each unit of a communication apparatus against a fading generated during the communication, each parameter is not always set to the optimal value, since the fluctuation speed of the fading varies depending on the moving speed of the mobile station (or fading pitch). Therefore, when the parameters are not optimized, the receiving characteristic is degraded, and thereby the channel capacity is reduced.
Since as described above, the DS-CDMA system comprises a searcher function, a transmission power control function, a coherent detection function, etc., it is necessary to dynamically modify the parameters of these functions and to reduce the influence of fading on received signals.
It is an object of the present invention to provide a communication control apparatus and method for setting each parameter for a communication apparatus to an optimal value in mobile communications using a DS-CDMA system, etc.
The communication control apparatus of the present invention comprises a speed estimation unit and a modification unit, and controls parameters for mobile communications between a transmitting station and a receiving station.
The speed estimation unit estimates the moving speed of either the transmitting station or the receiving station, and outputs a control signal corresponding to the estimated moving speed. The modification unit modifies the value of the parameters based on the control signal.
Each of the transmitting station and the receiving station corresponds to a base station or a mobile station in mobile communications. For example, the speed estimation unit estimates the moving speed of the receiving station using a transmission power control command transmitted from the receiving station to the transmitting station, and estimates the moving speed of the transmitting station using a desired signal power generated in the receiving station. In fact, since either of the base station and the mobile station has both functions of a transmitting station and a receiving station, either of the base station and the mobile station can estimate the moving speed of an opposing station using either the transmission power control command or desired signal power.
The modification unit modifies a variety of apparatus parameters for the transmitting and receiving stations based on a control signal outputted from the speed estimation unit. For these parameters, the frequency of a search operation for detecting a timing for despreading spread spectrum signals, the accumulation frequency of receiving correlation values in the search operation, the number and weight coefficient of pilot signals used in a coherent detection, an observation length for observing signals in an invalid path detection, the power value variation width and power value modification frequency in a transmission power control, etc. are considered. By controlling these parameters based on a control signal, communication control is available according to the moving speed, and thereby the receiving characteristic of the receiving station is improved.